The Correlation Between Shale Sealing Capacity And The Gas Generation Characteristics Of Shale Organic Matter,Minerals Compositions And Pore Structure

Because of the lacking of appropriate evaluation index of the mineability for shale gas according with the actual geological condition in China,China needs to develop appropriate evaluation index of the mineability for shale gas.The sealing capacity was an important evaluation index for shale gas exploriation.This study mainly discussed the shale sealing capacity based on the results of Hydrocarbon generation characteristics and pore structures,and tried to get the internal relations between shale sealing capacity and Hydrocarbon generation characteristics and pore structures.Further,this study tried to establise the evaluation index of shale sealing capacity suitable for the actual geological condition of marine shale block in southern china and use the evaluation index of shale sealing capacity to estimate the mineability of marine shale gas in southern china.Meanwhile,the pore structures and gas storage significance of the Wufeng Formation Shale from eatern Chongqing based on non-local density functional theory?NLDFT?with CO2-N2 adsorption data were also studied.These above studies have good theoretical significance to depper understanding the marine shale sealing capacity in china and benefit to the mass industrial production of shale gas.The topic of this paper was mainly to study the shale sealing capacity from the carbon isotopic rovoller and pore structure,three aspects were focused on.In order to simulate the generation and expulsion process under different sealing capacity,a calcareous shale sample of Neoproterozoic Xiamaling Formation from Xiahuayuan area of North China was selected for pyrolysis experiment in semi-closed system.All of these solids were then put in a pressurized closed experimental system?gold tube?to study the compositions and ?13C of gas generated from the secondary pyrolysis;The Wufeng-Longmaxi Formation shale samples with TOC of about 4% were selected as investigation target to ignore the role of organic matter?content,type and maturity?.These shale samples were measured of inorganic minerals compositions,the pore size distribution,the parameter for porosity and permeability,displacement pressure.Based on the above results,the effect of inorganic minerals compositions and pore structures on the shale capillary sealing capacity was systematically studied.Ten shale samples of Wufeng Formation from Shizhu area of East Sichuan Basin were selected as samples.The specific surface area and pore volume were obtained from the non-local density functional theory?NLDFT?.Finally,the pore structures and the significance of pore structure on the storage of shale gas were discussed.These results indicated that higher pressure inhibited the cracking of kerogens,and led to the decrease of the consumption of lipid organic carbon,especially the oil potential organic carbon.Higher pressure also led to the decrease of the EE,but only had slight influence on the content and character of residual bitumen?group component and ?13C?.During the secondary pyrolysis,the kerogens from the residual solids heated under the higher pressure in the primary pyrolysis tend to generate more oil and less gas and the ?13C of the secondary gas tend to be more negative.The sequence of the yields and ?13C of the secondary gas from the residual solids primarily at 370°C is as follows: at 50MPa>at 75 MPa>at 100 MPa.The carbon isotopic reversal of the gas hydrocarbons with the increase of temperature was occurred at 370°C in the primary pyrolysis.But there is no such kind of obvious reversal in the secondary pyrolysates.?13C of the secondary gas hydrocarbon increased slowly with the increase of maturity in the range of Easy Ro=1.52%-1.98%.It indicated that the mixture mechanism could affect the thermal evolution of carbon isotope compositions of shale gas.The effected extent is different at different mature stages.It is obvious at high and over mature stage and weak at low and middle mature stages.Although our experiment result did not show a carbon isotopic reversal of the gas hydrocarbons as obvious as previously reported in geological cases,it still could be a significative experiment evidence for the mixture mechanism of carbon isotopic reversal of shale gas.The results indicate that the pore structure directly affect the capillary sealing capacity.Namely,the large mesopore and macropore could increase permeability and decrease the capillary sealing capacity because of the better connectivity.Inorganic minerals affect the shale pore structure,then the shale capillary sealing capacity was effected by the pore structure.The inorganic minerals mainly effect the development of large mesopore?>10nm?and macropore,and slightly effect the development of micropore.Eventually,the shale with higher content of brittle minerals has lower capillary sealing capacity.The results provide reliable experiment evidence for the quantitative research of the effect of inorganic mineral compositions and pore structure on the self-sealing capacity.The results showed that the Wufeng Formation shales have very high content of brittle minerals.The brittleness index was in the range of 0.52-0.88 with the average of 0.71,which indicated that the shale was very easy to be fractured.The micropore was the most important sites of adsorbed shale gas,because it provided the major specific surface area for adsorbing shale gas with the average of specific surface area of 18.27 m²/g,and accounted for 81.61% of total specific surface area;The mesopore and macropore were the most important storage space of free shale gas,because they provided the major pore volume used for storing free shale gas with the average pore volume of 22.51×10^-3 cm³/g,and accounted for 81.48% of total pore volume.In Shizhu area of East Chongqing,the content of adsorbed shale gas by NLDFT model was significantly higher than that from previous assessments by other models.The content of free shale gas by NLDFT model was also higher than that from previous assessments by other models.